Method of fabrication of step edge

ABSTRACT

A method of forming a step edge in a surface  12  of a crystalline substrate  10 , comprising the steps of forming a layer of resist  11  over the surface  12  and removing areas of the resist  11  to expose selected areas of the surface  12 , thereby forming side walls  13  in the layer of the resist  11 , the side walls  13  bounding the exposed areas of the surface  12 . The method further comprises exposing the resist  11  and substrate  10  to an ion beam  14 , thereby etching the resist  11  and the exposed areas of the surface  12 , and controlling the orientation and angle of incidence of the ion beam  14  which respect to the substrate  10  and the resist side walls  13  to form a step edge with desired angle and height characteristics. An angular position of the substrate  10  about an axis  15  formed by a normal to the surface  12  is controlled in order to control the step edge formation.

TECHNICAL FIELD

The present invention relates to the construction of circuits includinga substrate, and in particular provides a method of forming a step edgein a substrate.

BACKGROUND ART

Superconducting materials are currently finding applications in a numberof areas. For example, superconducting quantum interference devices(SQUIDS) have applications in geophysical mineral prospecting.

In many instances it is desired to form features in the surface of asubstrate to alter or control physical or electrical aspects of circuitsconstructed over the substrate. For example, a ‘step edge’ in thesurface of the substrate is commonly required in the construction ofcircuits.

A common circuit element in superconducting devices is the JosephsonJunction. Which may be formed in a variety of ways. Josephson Junctionsare commonly implemented by forming the superconducting material over astep edge in a substrate. However, characteristics of the junction, suchas the critical density, can be difficult to control.

DISCLOSURE OF INVENTION

Throughout the following, the terms ‘superconducting material’,‘superconducting device’ and the like are used to refer to a material ordevice which, in a certain state and at a certain temperature, iscapable of exhibiting superconductivity. The use of such terms does notimply that the material or device exhibits superconductivity in allstates or at all temperatures.

From a first aspect the present invention provides a method of forming astep edge in a surface of a crystalline substrate, comprising the stepsof:

forming a layer of resist over the surface;

removing areas of the resist to expose selected areas of the surface,thereby forming side walls in the layer of the resist the side wallsbounding the exposed areas of the surface: and

exposing the resist and substrate to an ion beam, thereby etching theresist and the exposed areas of the surface;

wherein the angles between an axis of incidence of the ion beam and thesurface, and between the axis of incidence of the ion beam and theresist side walls, are selected in order to form a step edge withpredetermined angle and height characteristics, and wherein an angularposition of the axis of incidence of the ion beam about an axis formedby a normal to the surface is selected in order to control the step edgeformation.

The selection of the angular position of the axis of incidence of theion beam about the axis formed by the normal to the surface may be madein order to control or alter an etch rate of the substrate, for exampleby controlling or altering an incidence of the ion beam relative tochannels in a lattice of the crystalline substrate.

The resist side wall is preferably at an angle to the surface of thesubstrate of greater than 70 degrees, and even more preferably greaterthan 80 degrees. Most preferably, the resist side wall is substantially,perpendicular to the surface of the substrate.

The angle of the resist side wall may be optimised by controlling thesteps involved in formation of the layer of resist and the removal ofareas of the resist, namely hot plate temperature, pre-exposuredevelopment, exposure time, UV light intensity, post exposure bakingtemperature and development time.

In embodiments where the angle of the step edge is desired to be large,the method of the invention may further comprise the steps of:

orientating the ion beam such that the angle between the axis ofincidence of the ion beam and a plane of the resist side wall isminimised, thereby minimising an etch rate of the resist side wall; and

altering the angle between the axis of incidence of the ion beam and thesurface in order to control an etch rate of the substrate.

Alternative embodiments, in which the angle of the step edge is desiredto be low, may further comprise the steps of:

orientating the ion beam such that the angle between the axis ofincidence of the ion beam and a plane of the resist side wall issufficiently large to cause an etch rate of the resist side wall to beincreased; and

altering the angle between the axis of incidence of the ion beam and thesurface in order to control an etch rate of the substrate.

During a period of time in which the resist and substrate are exposed tothe ion beam, both the substrate and the resist side wall will be etchedby the ion beam. Consequently, the resist side wall will graduallyrecede from the exposed areas of the surface, thereby exposing furtherareas of the surface to the ion beam. The further areas of the substratesurface will be exposed to the ion beam for a reduced amount of time,and therefore the substrate will be less deeply etched in these areas,forming the step edge of a certain angle.

The angle of the step edge may be controlled in order to obtainpredetermined characteristics in a circuit subsequently constructed overthe substrate. For example, a high temperature superconductor may laterbe formed over the substrate, with a Josephson junction being formedover the step edge. By forming a step edge having a predetermined angle,the critical current of the Josephson junction may, to some extent. beselected.

The substrate used in preferred embodiments of the invention is a singlecrystal MgO (100) substrate.

The step of removing areas of the resist is preferably performed byphotolithography.

The resist and substrate are preferably exposed to an argon ion beam.

The method of the first aspect of the invention preferably comprises thepreliminary step of:

providing a smooth substrate surface, for example by polishing thesurface of the substrate.

The substrate preferably has a surface roughness of less than 0.4 nm.

The method of the first aspect of the invention preferably comprises thesubsequent steps of:

removing all of the resist; and

cleaning the surface of the substrate to smoothen irregularities in thesurface and to remove any debris that may have been created duringprevious steps.

A height of the step edge may be influenced by controlling a time forwhich the surface and resist are exposed to the ion beam.

Preferably, the method of the first aspect of the invention provides astep edge suitable for forming a Josephson junction in a superconductingmaterial, having a single grain boundary at the upper part of the stepedge, and a rounded step base.

The superconducting material is preferably YBa₂Cu₃O_(x) (YBCO), where xhas a value of 6 to 7. YBCO is advantageous in the present methodbecause it grows over an MgO substrate such that its c-axis remainssubstantially perpendicular to the underlying substrate, enabling grainboundaries to be created in the YBCO HTSC layer. Consequently, in caseswhere a Josephson junction is constructed over the step edge, thecritical current of the junction may be controlled by the angle of thestep edge, and by the number of misorientation angles that are formed inthe step.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows a substrate being exposed to an ion beam in accordance withthe first aspect of the present invention;

FIGS. 2 and 2a show views of the substrate of FIG. 1 before and afteretching;

FIGS. 3a to 3 e are scanning electron micrographs of the morphology ofstep edges formed in accordance with the present invention;

FIG. 4 illustrates the effect of simultaneous etching of the resist sidewall and the substrate; and

FIGS. 5a to 5 d are scanning electron micrographs of the morphology ofstep edges which have been cleaned by a final ion beam etch.

MODES FOR CARRYING OUT THE INVENTION

In accordance with a first aspect of the present invention, FIG. 1 showsa crystalline substrate 10 having a layer of resist 11 formed over asurface 12 of the substrate 10. Areas of the resist have been removed inorder to expose an area of the surface 12, and the resist has a sidewall 13 adjoining the exposed area of the surface 12. The substrate 10and resist 12 are being exposed to an argon ion beam 14 of controlledorientation and angle of incidence with respect to the surface 12 and tothe side wall 13, thereby etching the exposed areas of the surface 12and the resist side wall 13 at controlled rates. Appropriate control ofthe orientation and angle of incidence of the ion beam 14 and the periodof exposure, provides a step edge of desirable angle and height.

The present invention will now be described by way of example. We defineα to be the angle of the substrate 10 to the ion beam 14, and β to bethe angular position of the substrate 10 about an axis 15 formed by anormal to the plane of the surface 12.

As shown in FIG. 2, resist side wall 13 is at an angle χ of between 80°and 85° to the surface 12. This angle is preferably as close as possibleto 90°, and may be optimised by controlling the steps involved information of the layer of resist 11 and the removal of areas of theresist, namely hot plate temperature, pre-exposure development, exposuretime, UV light intensity, post exposure baking temperature anddevelopment time. Resist side walls at an angle of between 80° and 85°to surface 12 are routinely achieved in this way. The argon ion beam 14forms an angle γ with the normal 16 of the plane of the resist side wall13.

Substrate 10 is a MgO (100) substrate polished on one side, to have asurface roughness of better than 0.4 nm. A 1.3 μm photoresist mask 11(Shipley S98913—available from AWA Electronics, 8 Australia Ave,Homebush Bay, Sydney, Australia) is used to define the step edge. Theargon ion beam is produced by a Kaufman ion gun providing a beam voltageof 500 eV and a neutralised beam current of 22 mA/cm² on a water cooledsubstrate holder.

In order to form a step edge of desired angle ψ, the etching rate of thesubstrate 10 and the etching rate of the resist side wall 13 arecontrolled. Our measurements indicate that the etching rates of MgO(100) and Shipley S98913 photoresist vary with different incidentangles, α. From normal incidence (α=0°), the etching rate increasesuntil reaching a maximum at α=50°-60°, then decreases as larger glancingangles are approached. The initial increase in rate occurs as aconsequence of the increased probability of the Ar ion collisions givinga substrate atom a component of momentum directed away from the surface12. The reduction in etching rate at higher angles occurs because theincoming ion beam is spread over an increasingly larger surface area(the flux drops off as the cosine of the incidence angle). Also, theprobability of a purely elastic reflection of the primary beam isincreased at large angles of incidence.

There follows an example of the effect of altering α, while keeping 7close to the glancing angle (>70°), thereby causing a slow etch rate ofthe resist side wall 13. The etching rate of the MgO (100) substrate 10is increased by raising the ion beam 14 angle of incidence from zero upto 60°-70°. Note that γ remains substantially constant as α is changed.Therefore the etch rate of the resist side walls 13 is constant, at asmall value, and the etch rate of the MgO substrate 10 is altered.Consequently, it is possible to alter the ion milled surface step angleψ. FIG. 3 shows scanning electron micrographs of the step edgemorphology after the resist mask has been removed, for α varied from 30°to 70° in 10° increments. The steepness of the step edge increases withα up to 60-70°. A ‘rabbit ear’ is observed on the top side of the stepfor α=60-70°, which is due to redeposited material backsputtered fromthe substrate surface. This material is mostly polycrystalline and wouldhave a deleterious effect on subsequently grown layers such as YBCO, andso is preferably cleaned off (further described below).

In cases where a lower step edge angle ψ is desired, γ may be reduced.This is now described by way of example. It is possible to fabricate astep edge with a moderate angle ψ by aligning the substrate 10 so thatthe resist side wall 13 faces into the ion beam 14. Therefore the etchrate of the resist side wall 13 will be significant. With γ now beingsignificantly less than 90 degrees, any change in a will affect γ.Specifically, increasing α will decrease γ. As shown in FIG. 4, ion beametching in such an orientation leads to the resist side wall 13 drawingback as shown by arrow 20 as it is etched away, simultaneously with theetching of the substrate 10 in the (100) direction as shown by arrow 21.The etch rate of the substrate 10 depends on α, whereas the etch rate ofthe resist side wall 13 depends on γ. Following etching, the resist andsubstrate will have drawn back, as shown by dotted line 22 in FIG. 4.

The step angle ψ may be influenced by altering the etch rate of theresist side wall with respect to the etch rate of the substrate. Inaccordance with the present invention, this may be achieved by alteringβ, thereby altering γ while maintaining a constant α. Moderate stepangles ψ are achieved when the etch rate for the substrate 10 isapproximately equal to the etch rate for the resist side wall 13. Theangles of α/γ=50°/30°, 60°/20° or 70°/5° have been found to achieve astep angle of ψ=35°-40°. It has been noted that while an alteration of βdoes not change α, it does have a significant effect on the etch rate ofthe MgO crystalline substrate, but not on the etch rate of the resist,which may have an affect on the final result. For example if the etchingrate of the MgO substrate 10 is too low, it may be possible to etchentirely through the photoresist and therefore etch and damage areas ofthe surface 12 which were not intended to be etched. If the etching rateof the MgO substrate 10 is too high, the resist side wall 13 may notdraw back sufficiently to define a desired step angle.

This effect occurs because the etching rate increases at more obliqueangles of incidence to a crystal structure, and material is more rapidlyetched from side walls of channels formed by the crystal latticestructure.

As mentioned above, some substrate orientations lead to the formation ofundesirable backsputtered debris on the surface 12, particularly nearthe top of the step edge. Such debris may cause unwanted misorientationangles in the step edge, for example causing increased junction noise ina Josephson junction subsequently constructed over the step edge. Inpreferred embodiments of the invention this debris is removed aftercreation of the step edge is complete and the resist has all beencleaned off the substrate. For example, the debris may be removed by afinal ion beam etch, with the ion beam preferably being alignedperpendicular to the surface. Such a cleaning step may also improve thesurface roughness, to provide a smoother mounting surface upon whichcircuits may subsequently be built. Step edges which have undergone anion beam etch to remove debris and smoothen the surface are shown inFIG. 5.

The present method enables production of a step edge having one, two orthree junctions. In the preferred case, one junction is formed by makinga step edge with a rounded bottom. The step edge may form two junctionsby having a sharply defined edge at the top and bottom of the step.Alternatively, the present invention may provide a step edge havingthree junctions, wherein a ‘trench’ is formed along the bottom of thestep, as indicated in FIG. 3.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

What is claimed is:
 1. A method of forming a step edge in a surface of acrystalline substrate, comprising the steps of: forming a layer ofresist over the surface; removing areas of the resist to expose selectedareas of the surface, thereby forming side walls in the layer of theresist, the side walls bounding the exposed areas of the surface; andexposing the resist and substrate to an ion beam, thereby etching theresist and the exposed areas of the surface; wherein the angles betweenan axis of incidence of the ion beam and the surface, and between theaxis of incidence of the ion beam and the resist side walls, areselected in order to form a step edge with predetermined angle andheight characteristics, and wherein an angular position of the axis ofincidence of the ion beam about an axis formed by a normal to thesurface is selected in order to control the step edge formation.
 2. Themethod according to claim 1 wherein the selection of the angularposition of the axis of incidence of the ion beam about the axis formedby the normal to the surface is made in order to control or alter anetch rate of the substrate, by controlling or altering an incidence ofthe ion beam relative to channels in a lattice of the crystallinesubstrate.
 3. The method according to claim 1 wherein the resist sidewall is at an angle to the surface of the substrate of greater than 70degrees.
 4. The method according to claim 3 wherein the resist side wallis at all angle to the surface of greater than 80 degrees.
 5. The methodaccording to claim 4 wherein the resist side wall is substantiallyperpendicular to the surface of the substrate.
 6. The method accordingto claim 1 wherein the angle of the resist side wall is controlled bycontrolling the steps involved in formation of the layer of resist andthe removal of areas of the resist, namely hot plate temperature,pre-exposure development, exposure time, UV light intensity, postexposure baking temperature and development time.
 7. The methodaccording to claim 1 wherein, in order to create a step edge of largeangle, the method further comprises the steps of: orientating the ionbeam such that the angle between the axis of incidence of the ion beamand a plane of the resist side wall is minimized, thereby minimizing anetch rate of the resist side wall; and altering the angle between theaxis of incidence of the ion beam and the surface in order to control anetch rate of the substrate.
 8. The method according to claim 1 wherein,in order to create a step edge of small angle, the method furthercomprises the steps of: orientating the ion beam and a plane of theresist side wall is sufficiently large to cause an etch rate of theresist side wall to be increased; and altering the angle between theaxis of incidence of the ion beam and the surface in order to control anetch rate of the substrate.
 9. The method according to claim 1 whereinthe angle of the step edge is controlled in order to obtainpredetermined characteristics in a circuit subsequently constructed overthe substrate.
 10. The method according to claim 9 wherein the circuitexhibits high temperature superconductivity.
 11. The method according toclaim 9 wherein a Josephson junction is formed over the step edge. 12.The method according to claim 11 wherein the critical current of theJosephson junction is controllably altered by altering the angle of thestep edge.
 13. The method according to claim 1 wherein the substrate isa single crystal MgO (100) substrate.
 14. The method according to claim1 wherein the step of removing areas of the resist is performed byphotolithography.
 15. The method according to claim 1 wherein the resistand substrate are exposed to an argon ion beam.
 16. The method accordingto claim 1 wherein the method comprises the preliminary step of:providing a smooth substrate surface.
 17. The method according to claim16 wherein the smooth substrate surface is provided by polishing thesurface of the substrate.
 18. The method according to claim 1 whereinthe substrate has a surface roughness of less than 0.4 nm.
 19. Themethod according to claim 1 wherein the method comprises the subsequentsteps of: removing all of the resist; and cleaning the surface of thesubstrate to smoothen irregularities in the surface and to remove anddebris that may have been created during previous steps of the method.20. The method according to claim 1 wherein a height of the step edge isinfluenced by controlling a time for which the surface and resist areexposed to the ion beam.
 21. The method according to claim 1 wherein themethod provides a step edge suitable for forming a Josephson junction ina superconducting material.
 22. The method according to claim 1 whereinthe method provides a step edge suitable for forming a Josephsonjunction in a superconducting material, having a single grain boundaryat the upper part of the step edge, and a rounded step base.
 23. Themethod according to claim 1 wherein a superconducting material whichgrows with its c-axis substantially perpendicular to the substrate isdeposited over the substrate.
 24. The method according to claim 1wherein YBa₂Cu₃ Ox is deposited over the substrate, x being in the rangeof 6 to
 7. 25. The method according to claim 1 wherein a number ofmisorientation angles and the angle of the step edge control a criticalcurrent of a Josephson junction.